1. Field of the Invention
The invention relates to fittings and, more particularly, to fittings for connecting together sections of rigid metal tubes for conveying liquids under sanitary conditions.
2. Discussion of the Related Art
Fittings are well known for connecting together sections of metal pipes or tubes. Threaded fittings and compression fittings are the most commonly used for this purpose because they are relatively reliable and relatively inexpensive to fabricate and install.
Referring to FIG. 1, a conventional swaglok-type compression fitting 10 includes a recessed body 12, a locking nut 14, and a gripping ring 16. The recessed body 12 receives the end of a tube 18, and the gripping ring 16 is disposed between the end of the body 12 and an outer flange of the nut 14. In use, as the nut 14 is threaded onto the body 12, fingers on the end of the gripping ring 16 are forced into a wedged-shaped end portion 20 of the body 12 so as to grip the tube 18 and to seal against both the tube 18 and against the end portion 20 of the body 12.
The fitting 10 exhibits several drawbacks and disadvantages rendering it poorly suited for use in systems requiring sanitary operating conditions. For instance, because the actual seal point between the gripping ring 16, the tube 18, and the body 12 is located remote from the end of the tube 18, a relatively large crevice C is present in the fitting. Fluids may collect in this crevice, promoting bacterial growth.
Threaded joints are considerably stronger than compression joints. However, voids are formed between the threads which are even larger than the crevices formed in compression fittings. Fluid accumulation and bacteria growth therefore are even more problematic in fittings having threaded joints than in those having compression joints.
Sanitary fittings are available which are designed to eliminate crevices in the vicinity of the end of the tube, but all previously known sanitary fittings exhibit certain drawbacks and disadvantages.
Referring for instance to FIG. 2, one type of sanitary fitting 30 employs a "roll-on" ferrule as the sealing portion of the fitting. Specifically, tubes 32, 34 to be joined are expanded into grooves formed on the inner periphery of ferrules 36, 38 using rollers. A gasket 40 is placed between the ferrules 36, 38, and the ferrules 36, 38 are forced towards one another, using a clamp or threaded nut (not shown) to compress the gasket therebetween. When the fitting 30 is properly assembled, the ends of the tubes 32, 34 are sealed to the gasket 40 to eliminate voids or crevices between the tubes 32, 34 and the gasket 40 and to prevent fluid accumulation and bacteria growth.
The roll-on ferrule fitting 30 exhibits several drawbacks and disadvantages. For instance, it is relatively difficult to assemble because the ends of the tubes 32, 34 must be expanded into contact with the ferrules 36, 38 as part of the assembly process. Moreover, unless the gasket 40 is perfectly positioned to seal against the ends of the tubes 32, 34 rather than against the ends of the ferrules 36, 38, leakage can occur, particularly during temperature cycling, due to the formation of crevices C between the ferrules 36, 38 and the tubes 32, 34 as a result of temperature differentials between the ferrules and the tubes. Even if the crevices C do no extend across the axial length of the interface between the tube 32, 34 and the ferrules 36, 38, fluids still may accumulate in the crevices C, thereby leading to unsanitary conditions and promoting bacterial growth.
Referring to FIG. 3, structural constraints prohibit the use of roll-on ferrule-type fittings 30' with tubes 32', 34' having a diameter below 1 inch OD because the gasket 40', required to provide proper alignment between the adjacent tubes 32' and 34', is not available below 1 inch in size. Accordingly, if one were to attempt to use this style of fitting with tubes of relatively small size, i.e., 1/2 inch, a major crevice C" is formed at the ferrule 36, 38 which promotes fluid accumulation and bacterial growth. Of course, the potential still exists for the formation of a crevice C' similar to the crevice C as discussed above in connection with FIG. 2 and the accompanying disadvantages.
While other sanitary fittings are known, some of which solve one or more of the problems discussed above, all are relatively expensive to manufacture, and many are not easily usable with cut lengths of standard tubing.
Another problem associated with many fittings is that they fail at relatively low pressures because the holding forces applied to the tubes are relatively weak. For instance, U.S. Pat. No. 2,158,757 to Kuestermier discloses a fitting in which the tube is held in place by a gripping ring. The gripping ring has an annular portion spaced from the tube and a plurality of identical finger portions sloping inwardly and downwardly to terminate closely adjacent the outer wall of the tube. The fingers grip the tube when the fitting is assembled and locked in place, thereby preventing movement of the tube relative to the fitting. However, the combined area of the fingers is somewhat smaller than the combined area of the gaps between the fingers, and only the tips of the fingers grip the tube. Accordingly, very high stresses are imposed on these fingers, even at relatively low pressures. When the pressure in the tube exceeds a relatively low threshold, the fingers will buckle or bend back over themselves and allow the tube to be forced out of the fitting under fluid pressure therein. Experiments and manufacturers' specifications have revealed that most fittings of this type fail at pressures on the order of 200-500 psi.